Styrene-isoprene three-armed high load bearing capacity block copolymer composition for adhesives

ABSTRACT

An adhesive which contains a tackifying resin and a branched three-armed block copolymer composition comprised of polymeric blocks wherein the molecular weight is from about 180,000 to about 250,000, the polystyrene content is from about 17 to about 23% by weight of the block copolymer composition and the percentage of S-I arms which are load bearing is greater than about 85%.

BACKGROUND OF THE INVENTION

This invention relates to a novel block copolymer composition for use inadhesives. More particularly, it relates to branched three-armedstyrene-isoprene block copolymer compositions comprised of polymericblocks and adhesives made using such compositions.

It is known that a block copolymer can be obtained by an anioniccopolymerization of a conjugated diene compound and an alkenyl arenecompound by using an organic alkali metal initiator. Block copolymershave been produced which comprise primarily those having a generalstructure

    A--B and A--B--A

wherein the polymer blocks A comprise thermoplastic polymer blocks ofalkenyl arenes such as polystyrene, while block B is a polymer block ofa conjugated diene such as polyisoprene. The proportion of thethermoplastic blocks to the elastomeric polymer block and the relativemolecular weights of each of these blocks is balanced to obtain a rubberhaving unique performance characteristics. When the content of thealkenyl arene is small, i.e. 5-30% the produced block copolymer is aso-called thermoplastic rubber. In such a rubber, the blocks A arethermodynamically incompatible with the blocks B resulting in a rubberconsisting of two phases--a continuous elastomeric phase (blocks B) anda basically discontinuous hard, glass-like plastic phase (blocksA)--called domains. Since the A--B--A block copolymers have two A blocksseparated by a B block, domain formation results in effectively lockingthe B blocks and their inherent entanglements in place by the A blocksand forming a network structure at normal use temperature.

These alkenyl arene domains act as physical crosslinks anchoring theends of many block copolymer chains. Such a phenomenon allows theA--B--A rubber to behave like a conventionally vulcanized rubber in theunvulcanized state and is applicable for various uses. For example,these network forming polymers are applicable for uses such as inadhesive formulations; in molded shoe soles; as an impact modifier forpolystyrene resins and engineering thermoplastics; for modification ofasphalt; etc.

Styrenic block copolymers are widely used to make pressure sensitiveadhesives for a wide variety of uses including tapes, labels and productassembly applications. Pressure sensitive adhesives based on styrenicblock copolymers such as KRATON® thermoplastic rubber provide excellentadhesion to a wide variety of substrates, including paper, cardboard andother cellulosic materials. For instance, U.S. Pat. Nos. 3,239,478 and3,935,338, which are herein incorporated by reference, disclose how tomake pressure sensitive adhesives of this type. They include examples oflinear S--I--S polymers, the latter at col.5 describes a polymer with apolystyrene content of 14% and a molecular weight of 145,000. Highholding power in a shear test is important in such adhesives becausethis is a function of the load bearing capability of the adhesive, animportant feature in packaging applications, for example. Thus, it wouldbe advantageous to have available an adhesive with higher shear holdingpower than is presently available and also processable as athermoplastic (melt viscosity less than 500 Pa-s at 177° C.).

SUMMARY OF THE INVENTION

The present invention relates to branched three-armed styrene-isopreneblock copolymer compositions comprised of polymeric blocks. Thesecompositions can be used in pressure sensitive adhesives. Thecompositions are characterized in that the percentage of S--I arms whichare load bearing must be greater than 85% to give the polymer highcohesive strength. The polystyrene content must be from 17 to 23% toinsure adequate adhesion properties. The polystyrene equivalentmolecular weight (M_(s)) as measured by Gel Permeation Chromatography(GPC) must be 180,000 to 250,000 to insure proper viscosity in theadhesive. The present invention also encompasses a pressure sensitiveadhesive composition comprising the above block copolymer compositionand a tackifying resin.

DETAILED DESCRIPTION OF THE INVENTION

As is well known, polymers containing both aromatic and ethylenicunsaturation can be prepared by copolymerizing one or more polyolefins,particularly a diolefin, in this case isoprene, with one or more alkenylaromatic hydrocarbon monomers, in this case styrene. The copolymers may,of course, be random, tapered, block or a combination of these, in thiscase block. The blocks in each arm of the copolymers of this inventionare linear.

Polymers containing ethylenic unsaturation or both aromatic andethylenic unsaturation may be prepared using free-radical, cationic oranionic initiators. Such polymers may be prepared using bulk, solutionor emulsion techniques. In any case, the polymer containing at leastethylenic unsaturation will, generally, be recovered as a solid such asa crumb, a powder, a pellet or the like. Polymers containing ethylenicunsaturation and polymers containing both aromatic and ethylenicunsaturation are, of course, available commercially from severalsuppliers.

Polymers of conjugated diolefins and copolymers of one or moreconjugated diolefins and one or more alkenyl aromatic hydrocarbonmonomers such as branched (S--I)₃ block copolymers are frequentlyprepared in solution using anionic polymerization techniques. Thebranched block copolymers may have three arms with the followingstructure where X is a coupling agent:

    (SI).sub.3 --X

This type of branched polymer may also be made without a coupling agent.Such polymers can be made with trifunctional initiators such that allthree arms of the polymer grow outwardly from the initiator. Suchpolymers have the formula:

    (SI).sub.3

The S in the above formula usually stands for a polystyrene block butother alkenyl aromatic hydrocarbon monomers may be used including vinylaryl compounds such as various alkyl-substituted styrenes,alkoxy-substituted styrenes, 2-vinyl pyridine, 4-vinyl pyridine, vinylnaphthalene, alkyl-substituted vinyl naphthalenes and the like. Forsimplicity herein, the terms styrene, polystyrene content andpolystyrene equivalent molecular weight are used but such terms areintended to include these other alkenyl aromatic hydrocarbons. Theconjugated diene which is used herein must be isoprene and the type ofisoprene polymerization technique which must be used is that in whichthe stereochemistry is adjusted so that predominantlycis-1,4-polyisoprene having a glass transition temperature of less than-50° C. as measured by differential scanning calorimetry at a 10° C. perminute temperature scan rate is produced. This type of polyisoprenepolymerization should be used because otherwise the polymer may not becompatible with the tackifying resin.

In general, when solution anionic techniques are used, such (S--I)₃block copolymers are prepared by contacting the monomers to bepolymerized simultaneously or sequentially with an organoalkali metalcompound in a suitable solvent at a temperature within the range fromabout -100° C. to about 150° C., preferably at a temperature within therange from about 0° C. to about 100° C. Particularly effective anionicpolymerization initiators are organolithium compounds having the generalformula:

    RLi.sub.n

wherein:

R is an aliphatic, cycloaliphatic, aromatic or alkyl-substitutedaromatic hydrocarbon radical having from 1 to about 20 carbon atoms; andn is an integer of 1 to 3.

In general, any of the solvents known in the prior art to be useful inthe preparation of such polymers may be used. Suitable solvents includestraight- and branched-chain hydrocarbons such as pentane, hexane,heptane, octane and the like, as well as alkyl-substituted derivativesthereof; cycloaliphatic hydrocarbons such as cyclopentane, cyclohexane,cycloheptane and the like, as well as alkyl-substituted derivativesthereof; aromatic and alkyl-substituted aromatic hydrocarbons such asbenzene, toluene, xylene and the like; hydrogenated aromatichydrocarbons, such as tetralin, decalin and the like. Linear and cyclicethers such as dimethyl ether, methyl ethyl ether, anisole,tetrahydrofuran and the like may be used in small amounts.

As described in U.S. Pat. No. 4,096,203, the disclosure of which isherein incorporated by reference, the styrene may be contacted with theinitiator. Next, the living polymer in solution is contacted withisoprene. The resulting living polymer has a simplified structureA--B--Li. It is at this point that the living polymer is coupled.Coupling is normally only used when a monofunctional initiator is used.

There is a wide variety of coupling agents that can be employed. Anypolyfunctional coupling agent which contains three reactive sites can beemployed. Examples of the types of compounds which can be used includethe polyepoxides, polyisocyanates, polyimines, polyaldehydes,polyketones, polyanhydrides, polyesters, polyhalides, and the like.These compounds can contain two or more types of functional groups suchas the combination of epoxy and aldehyde groups, isocyanate and halidegroups and the like. Various other substituents which are inert in thecoupling reaction can be present such as hydrocarbon radicals asexemplified by the alkyl, cycloalkyl, aryl, aralkyl and alkaryl groupsand the alkoxy, aryloxy, alkythio, arylthio, and tertiary amino groups.Many suitable types of these polyfunctional compounds have beendescribed in U.S. Pat. Nos. 3,595,941; 3,468,972, 3,135,716; 3,078,254,and 3,594,452, the disclosures of which are herein incorporated byreference. When the coupling agent has two reactive sites such asdibromoethane, the polymer will have a linear AB--BA structure. When thecoupling agent has three reactive sites, such astrisnonylphenylphosphite (TNPP), the polymer will have a branchedstructure, such as (A--B)₃ --P. Since the present block copolymers arepredominantly branched with three arms, a coupling agent with threereactive sites must be used. Other phosphite coupling agents which canbe used include trimethyl phosphite and triethyl phosphite. Silanecoupling agents which can be used include methyl trichlorosilane, methyltrimethoxy silane and γ-glycidoxy propyl trimethoxysilane.

In the prior art, such as that exemplified by U.S. Pat. Nos. 3,595,941and 3,468,972 the effort was always made to select the particularcoupling agent or reaction conditions that resulted in the highestcoupling efficiency. High coupling efficiency is desired herein in orderto produce strong adhesive compositions. Coupling efficiency is definedas the mass of coupled polymer divided by the mass of coupled polymerplus the mass of uncoupled polymer. The coupling efficiency hereinrefers to that of the original polymer not including any degradationfragments formed during processing. Thus, when producing an (SI)₃branched polymer, the coupling efficiency is shown by the followingrelationship: ##EQU1## Coupling efficiency can be determinedtheoretically from the stoichiometric quantity of coupling agentrequired for complete coupling or coupling efficiency can be measured byan analytical method such as gel permeation chromatography. Typicalprior art coupling efficiency is from about 80% to almost 100%. In U.S.Pat. No. 4,096,203, coupling efficiency is controlled from about 20% toabout 80%, preferably about 30% to about 70%.

Less than maximum coupling efficiency can be achieved by a number ofmethods. One method to reduce coupling efficiency is to add less thanthe stoichiometric amount of coupling agent required for completecoupling of the polymers. Another means of reducing coupling efficiencyis by the premature addition of a terminator compound. Theseterminators, such as water or alcohol, react very quickly and couldeasily be employed to cut short complete coupling of the polymers. Inaddition, by performing the coupling reaction at elevated temperatures,such as above about 90° C., thermal termination of many of the livingpolymer groups (A--B--Li) occurs prior to coupling. The typical couplingconditions include a temperature of between about 65° C. to about 75° C.and sufficient pressure to maintain the reactants in a liquid phase.

As stated above, polymers within the scope of the present invention canalso be made without coupling by utilizing a trifunctional initiator. Inthis type of reaction, the polymer arms grow outwardly from theinitiator and one polymer is made rather than making more than onepolymer and coupling them together. Suitable trifunctional initiatorsfor use herein are prepared from the reaction of one equivalent of1,3,5-tris (1-phenylethenyl) benzene with three equivalents of sec-butyllithium according to the procedures described in "New Anionic Synthesesof Star-Branched Polymers" by R. P. Quirk, S-H Guo, M. Alsamarraie andF. Ignatz-Hoover, Polymer Preprints, Volume 29, pages 298-299 (Sep.1988) and "Dilithium Initiatiors Based on 1,3,5-bis (1-phenylethenyl)benzene. Tetrahydrofuran and Lithium sec--Butoxide Effects" by R. P.Quirk and J-J. Ma, Polymer International, 24, pages 197-206 (1991) fordifunctional initiators.

Following the coupling reaction or when the desired coupling efficiencyhas been obtained or when the trifunctional initiator - initiatedreaction is complete, the product is neutralized such as by the additionof terminators, e.g. water, alcohol or other reagents, for the purposeof removing the lithium radical forming the nucleus for the condensedpolymer product. The product is then recovered such as by coagulationutilizing hot water or steam or both.

As discussed above, it is desired that the pressure sensitive adhesiveformulation exhibit good adhesion, i.e. a rolling ball tack of less than5 cm, and have a melt viscosity appropriate for such adhesives, i.e.less than 500 Pa-s, and a shear holding power to Kraft of greater than2000 minutes. The present invention provides a composition and anadhesive which satisfies these requirements and is particularly usefulin packaging applications. The composition is a branched three armed(S--I)₃ block copolymer composition which is comprised of polymericblocks combined with a tackifying resin. In order to achieve thestandards set forth above, the polymer compositions of the presentinvention must have the following characteristics:

(1) a percentage of S--I arms which are load bearing of greater than85%,

(2) a polystyrene content (PSC) of 17 to 23% by weight, and

(3) a molecular weight (M_(s) --polystyrene equivalent peak molecularweight measured by gel permeation chromatography) of 180,000 to 250,000.Alkenyl aromatic hydrocarbons other than styrene may be used but forconvenience herein, this molecular weight will be referred to aspolystyrene equivalent molecular weight throughout.

It has been determined that if the compositions do not have the abovecharacteristics, then the adhesion, melt viscosity and high shearholding power criteria cannot be met. However, if the composition doeshave the above characteristics, then the adhesion, melt viscosity andhigh shear holding power criteria are satisfied.

If the polystyrene content is less than 17%, the shear holding power ofthe adhesives made from the composition is poor and if it is more than23%, the tack is too low. If the total molecular weight is greater than250,000, then the melt viscosity is too high and if it is less than180,000, the shear holding power is too low.

The percentage of S--I arms which are load bearing refers to thepercentage in the polymer before it is processed into a hot meltadhesive. Any polyisoprene-containing block polymer that does not have apolystyrene block on at least the two ends is not load bearing, i.e. itwill not contribute to the high shear holding power and high cohesioncharacteristics of the polymer. Simplifying somewhat, if the polymer hasa percentage of S--I arms which are load bearing of 90%, this means that90% of the polymer is made up of S--I--S linkages or some other suchlinkage in which the styrene is present on at least the two ends. When apolymer is made by the coupling process, the coupling efficiency of thepolymer is the percentage of S--I arms which are load bearing. Sincetrifunctional initiated polymers are not made by coupling, couplingefficiency has no meaning in that context. In the case of trifunctionalinitiated polymers, the percentage of S--I arms which are load bearingis the percentage of polyisoprene arms which become capped withpolystyrene blocks in the polymer synthesis.

The concentration of the initiator can be regulated to control themolecular weight of the overall composition and of the polystyreneblocks. Generally, the initiator concentration is in the range of about0.25 to about 50 millimoles per 100 grams of styrene monomer. Therequired initiator level frequently depends upon the solubility of theinitiator in the hydrocarbon diluent. The ratio of the initiator to themonomer determines the block size, i.e. the higher the ratio ofinitiator to monomer the smaller the molecular weight of the block.

Methods of controlling the molecular weights of the blocks and theoverall polymer are quite well known. For instance, such are disclosedin U.S. Pat. No. 3,149,182, which states that the amount of monomer canbe kept constant and different molecular weights can be achieved bychanging the amount of catalyst or the amount of catalyst can be keptconstant and different molecular weights can be achieved by varying theamount of the monomer, and in U.S. Pat. No. 3,231,635, the disclosuresof which are herein incorporated by reference, and many others. Atypical 3-armed block copolymer composition within the scope of thepresent invention, having a coupling efficiency of 86%, a polystyreneblock molecular weight of 12,000, a polystyrene content of 21% by weightand an overall molecular weight (M_(s)) of 226,000, was prepared bypolymerizing styrene with secondary butyl lithium as initiator at amonomer to initiator molar ratio of 115 to 1, polymerizing isoprene withthis polystyryl lithium as initiator at a monomer to initiator molarratio of 672 to 1 and then coupling this polystyrene-polyisoprenyllithium diblock with a trifunctional coupling agent.

It is necessary to add an adhesion promoting or tackifying resin that iscompatible with the elastomeric conjugated diene block. A commontackifying resin is a diene-olefin copolymer of piperylene and2-methyl-2-butene having a softening point of about 95° C. This resin isavailable commercially under the tradename Wingtack® 95 and is preparedby the cationic polymerization of 60% piperylene, 10% isoprene, 5%cyclopentadiene, 15% 2-methyl-2-butene and about 10% dimer, as taught inU.S. Pat. No. 3,577,398. Other tackifying resins of the same generaltype may be employed in which the resinous copolymer comprises 20-80weight percent of piperylene and 80-20 weight percent of2-methyl-2-butene. The resins normally have softening points (ring andball) between about 80° C. and about 115° C. ("Standard Test Method forSoftening Point by Ring-and-Ball Apparatus"; reference: 1989 Annual Bookof ASTM Standards, Volume 6,D3, page 520--ASTM Number E28- 67, 1982).

Other adhesion promoting resins which are also useful in thecompositions of this invention include hydrogenated rosins, esters ofrosins, polyterpenes, terpenephenol resins and polymerized mixedolefins. To obtain good thermo-oxidative and color stability, it ispreferred that the tackifying resin be a saturated resin, e.g., ahydrogenated dicyclopentadiene resin such as Escorez® 5000 series resinmade by Exxon Chemical Co. or a hydrogenated polystyrene orpolyalphamethylstyrene resin such as Regalrez® resin made by Hercules,Inc.

The amount of adhesion promoting resin employed varies from about 50 toabout 200 parts by weight per hundred parts rubber (phr), preferablybetween about 100 to about 150 phr. The selection of the particulartackifying agent is, in large part, dependent upon the specific blockcopolymer employed in the respective adhesive composition.

The adhesive composition of the instant invention may containplasticizers, such as rubber extending plasticizers, or compounding oilsor liquid resins. Rubber compounding oils are well-known in the art andinclude both high saturates content oils and high aromatics contentoils. Preferred plasticizers are highly saturated oils, e.g. Tufflo®6056 oil made by Arco Chemical Company, and oils with relatively lowaromatics content, e.g. SHELLFLEX® 371 oil made by Shell Oil Company.

Optional components of the present invention are stabilizers whichinhibit or retard heat degradation, oxidation, skin formation and colorformation. Stabilizers are typically added to the commercially availablecompounds in order to protect the polymers against heat degradation andoxidation during the preparation, use and storage of the adhesivecomposition.

Additional stabilizers known in the art may also be incorporated intothe adhesive composition. These may be for protection during the usefullife of the product against, for example, oxygen, ozone and ultra-violetradiation. However, these additional stabilizers should be compatiblewith the essential stabilizers mentioned herein-above and their intendedfunction as taught herein.

The adhesive compositions of the present invention are typicallyprepared by blending the components at an elevated temperature,preferably between about 130° C. and about 200° C., until a homogeneousblend is obtained, usually less than three (3) hours. Various methods ofblending are known to the art and any method that produces a homogeneousblend without undue degradation is satisfactory.

The resultant adhesives may be used in a wide variety of pressuresensitive applications. A particularly preferred application is theiruse in packaging tapes. Other uses include a wide variety of tapes,labels and assembly adhesives. Another important advantage of thecompositions of the present invention is that they give much bettershear holding power than similarly made polymers which also meet theindustry standards. This is shown in the examples.

In the following examples, the coupling efficiency was determined byGPC. The melt viscosity (M.V.) was measured in Pascal-seconds (Pa-s) byusing a Brookfield Thermocell viscometer at 177° C. The SAFT Mylar(SAFT-MY) was measured by 1"×1" Mylar to Mylar lap joint with a 1 kgweight. The SAFT Kraft (SAFT-KT) was measured by 1"×1" Mylar to Kraftpaper lap joint with a 1 kg weight. SAFT measures the temperature atwhich the lap shear assembly fails underload in an oven whosetemperature is raised at a rate of 40° F. per hour. The molecularweights were peak molecular weights determined by gel permeationchromatography as polystyrene equivalent molecular weight. Thepolystyrene content was determined by nuclear magnetic resonancespectroscopy. Rolling Ball Tack (RBT) is the distance in centimeters asteel ball rolls on the adhesive film with a standard initial velocity(PSTC test No. 6). Small numbers indicate aggressive tack. Holding Powerto steel (HP-ST) or to Kraft paper (HP-KT) is the time required to pulla standard area (1/2"×1/2") of tape from a standard test surface (steel,Kraft paper) under a standard load (2 kg), in shear at 2° antipeel (PSTCtest No. 7). Peel was determined by PSTC test No. 1. Polyken probe tack(PPT) was determined by ASTM D-2979. Loop tack (LT) was determined usingTLMI loop tack tester. For HP, Peel, PPT and LT, higher numbers indicatebetter performance for most pressure sensitive adhesive applications.

EXAMPLE

Polymer A is a polymer commonly used in pressure sensitive adhesiveapplications. It is a (SI)₂ --X block copolymer with a couplingefficiency of 82%, a polystyrene content of 14.8% and a molecular weight(MW_(s)) of 220,000. Polymer B is a polymer made according to thepresent invention by using TNPP as a coupling agent. It is a three-armedbranched block copolymer wherein the arms are linear S--I blocks. This(S--I)₃ --X poly a coupling efficiency of 86%, a polystyrene content of21% by weight and a molecular weight (MW_(s)) of 228,000. Both polymerswere used in the adhesive formulation shown at the bottom of Table 1.Adhesive formulations in Table 1 were dissolved in reagent gradetoluene, cast onto 1.0 mil MYLAR® film to a dried adhesive thickness ofapproximately 1.5 mils using a doctor blade. Adhesive testing wasconducted on dried adhesives according to the methods described above.It can be seen that while most of the adhesive properties of theformulation using Polymer B are somewhat better than those of the otherformulation (RBT is somewhat less desirable, probably because of thehigher styrene content), the HP-ST is dramatically increased.

                  TABLE 1                                                         ______________________________________                                                    POLYMER                                                           PSA PROPERTY    A         B                                                   ______________________________________                                        RBT, CM         .9        1.5                                                 PPT, KG         1.45      1.74                                                LT, OZ/IN       65        78                                                  PEEL, PLI       4.7       5.0                                                 HP-ST, MIN      561       >12000                                              HP-KT, MIN       1737 pc/a.sup.1                                                                         8989 pc/a.sup.1                                    SAFT-MY, °C.                                                                           101.sup.2 c                                                                             108.sup.2 c                                         SAFT-KT, °C.                                                                           77.sup.3 a                                                                              83.sup.3 a                                          M.V., Pa.S      140       270                                                 ______________________________________                                         .sup.1 pc/a means partially cohesive/adhesive.                                Pa · s? 2c means cohesive failure.                                   .sup.3 a means adhesive failure.                                              Formulation:                                                                  100 parts polymer                                                             100 parts tackifying resin (ESCOREZ 1310 from Exxon Chemical Co.)             10 parts naphthenic oil (SHELLFLEX 371 from Shell Oil Co.)                    1 part phenolic antioxidant (IRGANOX 1010 from CibaGeigy Corp.)          

We claim:
 1. A branched three-armed S--I--S block copolymer composition,where S is an alkenyl aromatic hydrocarbon and I is isoprene, comprisedof polymeric blocks for use in pressure sensitive adhesives, said blockcopolymer composition characterized in that(a) the molecular weight (aspolystyrene equivalent) is from 180,000 to 250,000, (b) the polystyrenecontent is from 17 to 23% by weight of the block copolymer compositionand (c) the percentage of S--I arms which are load bearing by weight ofthe block copolymer composition is greater than 85%.
 2. The compositionof claim 1 wherein S is styrene.
 3. A pressure sensitive adhesivecomprising the block copolymer composition of claim 1 and a tackifyingresin.
 4. A predominantly branched S--I--S blockcopolymer composition,where S is an alkenyl aromatic hydrocarbon and I is isoprene, comprisedof polymeric blocks for use in packaging adhesives which have a rollingball tack of less than 5 centimeters, a melt viscosity of less than 500Pa-s at 177° C. and a shear holding power to Kraft of greater than 2,000minutes, said block copolymer composition characterized in that(a) themolecular weight (as polystyrene equivalent) is from 180,000 to 250,000,(b) the polystyrene content is from 17 to 23% by weight of the blockcopolymer composition and (c) the percentage of S--I arms which are loadbearing by weight of the block copolymer composition is greater than85%.
 5. The composition of claim 4 wherein S is styrene.
 6. An adhesivecomprising the block copolymer composition of claim 4 and a tackifyingresin.